1. Field of the Invention
This invention relates to acoustic range finding systems of the type in which an electro-acoustic transducer transmits a pulse or shot of acoustic energy towards a surface whose distance is to be measured, and subsequent signals received from the transducer are monitored to determine the temporal location of an echo from that surface.
2. Review of the Art
In such a system, various compromises are necessary in accommodating the characteristics of the transducer and its associated transmitter and receiver. The reactances of the transducer and of circuits coupling the transducers to the transmitter and receiver form a tuned circuit resonant at a frequency at or near the operating frequency of the transducer, and the quality factor or Q of this circuit has a profound effect upon the amplitude of the acoustic energy generated, the sensitivity of the transducer to return echoes, and the amplitude and rate of decay of the "ringing" of the transducer following transmission of the shot. The higher the Q, the greater the amplitude and sensitivity, but also the greater the ringing, which may severely affect the ability of the system to detect close-in echoes since these may either be swamped by the ringing, or rendered undetectable through saturation of the receiver caused by high amplitude ringing. In actual systems such as those described in commonly assigned prior U.S. Pat. Nos. 4,831,565; 4,890,266; 4,992,998 and 5,079,751, the input of the receiver is protected against high amplitude signals from the transmitter by placing opposite polarity pairs of limiting diodes at the input of the receiver, in series with a limiting resistor or resistors, such that during operation of the transmitter the diodes conduct. The limiting resistor acts both to protect the receiver, and to prevent too large a proportion of the transmitter energy being dissipated in the diodes. The resistor or resistors also apply some limited damping (i.e. reducing the Q) to the transducer circuit. During reception of low amplitude echo signals, the diodes cease to conduct, thus placing the resistor or resistors in series with a higher impedance or impedances presented by the receiver input and somewhat reducing the damping so as to provide improved sensitivity and noise immunity during reception. Whilst this arrangement works well, it still requires design compromises. The ringing characteristics of the transducer circuit are difficult to control closely due to varying characteristics of individual transducers, the reactance and resistance of varying lengths of transmission line connecting them to the transmitter, and imperfections of the impedance matching transformer usually located between the active element of the transducer and the transmission line. These factors mean that the limiting resistor or resistors can exercise only a small degree of control over damping of the transducer. Sufficient damping must be applied to ensure that clipping by the diodes will cease and the receiver will be unsaturated within a time, following the start of a shot, corresponding to the minimum range to be measured. Whilst various techniques, such as shortened shots, may be used to provide a short minimum range, the receiver requires a very large dynamic range to accommodate the extreme differences in signal amplitude existing between short range and distant echoes. This in turn has tended to require the use of arrangements such as those disclosed in U.S. Pat. No. 4,596,144 (Panton et al.), using multiple logarithmic amplifiers, to provide the necessary dynamic range. An alternative approach has been to place an electronically switchable attenuator at the input to the receiver which is switched in for short range shots only in order to reduce the dynamic range required in the receiver. Such an arrangement is used in the MULTIRANGER (trade-mark) system of the assignee of this application.
Yet another approach has been to utilize variable gain control of the receiver; see for example U.S. Pat. No. 4,439,846 (Rodriguez) which represents a development of the Muggli patent discussed below. Satisfactory wide range gain control can present considerable implementation problems and leaves the necessity for the receiver to cope with a very wide range of signal amplitudes at its input.
U.S. Pat. No. 4,199,246 (Muggli), issued Apr. 22, 1980, describes an ultrasonic ranging system in which the transmitter is driven by a voltage controlled oscillator, such that the frequency transmitted by the transducer is changed in a predetermined manner over a substantial range during the course of the transmitted pulse. The bandwidth of the receiver is varied, again according to a preset pattern, during a subsequent period by changing the receiving Q so that the receiver bandwidth is narrowed with the passage of time following the pulse, the passband of the receiver being centred upon the lowest frequency transmitted. By configuring the transmitted pulse so that a short initial portion is transmitted at a relatively high frequency, which is then reduced in one or more steps to a relatively low frequency, and configuring the receiver so that its initial bandwidth is wide enough to pass the highest as well as the lowest frequency, short range echoes of the high frequency pulse components may be detected, but at longer ranges, reception of the low frequency component and exclusion of noise is optimized, by decreasing the bandwidth and thus improving the quality factor (Q) of the receiver. The variable Q circuit used in the receiver only however affects the receiver circuits, and does not vary the damping applied to the transducer itself, whose characteristics remain unmodified. The dynamic range of the signals appearing at the receiver input is thus unmodified.
The Muggli system is subject to two constraints which limit its applicability. The transducer itself must be capable of operation over a wide range of frequencies, and the noise immunity of the system at short ranges is very poor because of the wide bandwidth of the receiver at those frequencies. Neither of these limitations need be serious in the camera control applications for which the Muggli system is clearly primarily designed, involving as they do low power transducers, comparatively short ranges, and environments which are comparatively noise-free at the frequencies of interest; they are however highly significant in typical industrial applications for which suitable transducers operating over a wide frequency range are not generally available. Instead it has been necessary to select a suitable transducer, and to provide a transmitter/receiver system whose frequency characteristics and output voltage are matched to the transducer.
U.S Pat. No. 5,079,751 discloses a control unit for connection to at least one electroacoustic transducer to form an acoustic ranging system, comprising a transmitter for generating shots of alternating current electric energy for application to each said transducer to generate acoustic energy, a tuned receiver for receiving and amplifying alternating current generated by each said transducer responsive to the receipt of acoustic energy, means for digitizing output from said receiver, and a control computer controlling said transmitter to time said shots and for processing said digitized receiver output to recognize therein features indicative of a primary echo from a target being ranged, said unit further including first electronically controlled means for determining an operating frequency of said transmitter, and second electronically controlled means for causing the tuning of said receiver to track the operating frequency of said transmitter, and said control computer further controlling said first electronically controlled means to determine the frequency of the transmitter during each shot responsive to data relative to characteristics of each said transducer.
As well as the frequency related control mentioned above, the patent also discloses how a main loading resistor connected in parallel with the transmitter can be associated with an additional resistor which is switched into parallel with the main loading resistor to adjust somewhat the damping of the transducer and thus its Q or quality factor. The optimum amount of damping may of course vary according to the transducer utilized, hence my incorporation of means to modify the damping in the unit which forms the subject of the above application, according to the characteristics of the transducer utilized during each shot.
In practice the additional resistor has, as already discussed, only a limited effect on the ringing characteristics of a transducer, since a typical transducer in a multitransducer array such as is contemplated in U.S. Pat. No. 5,079,751 will necessarily be in most instances remote from the transceiver. Transducers suitable for such application incorporate a built-in transformer so that the connecting cable to the transducers may operate at relatively low impedance, and the combination of the imperfections of the built-in transformer and the reactance of the cables will mean that only limited damping can be applied by the additional resistor even if its value is so low as to severely limit the transmitter output. The arrangement is thus only suitable for achieving such minor adjustments of ringing characteristics as are necessary to suit different transducers.
With the continuing decrease in the cost of implementing even quite sophisticated electronic digital signal processing, it becomes increasingly practical to carry out echo processing at a location adjacent the transducer, the resulting range information being monitored at a remote location. It has been known for some time to provide local preprocessing of echo data adjacent the transducer, for example from published U.K. Patent Application 2151357A (Endress & Hauser GmbH & Co.), in order to facilitate transmission of the data to a remote processing unit, and more recently systems have appeared in which the data processing also has been carried out adjacent the transducer.